Are the three accounts with the same provider (the same email company) or with different ones? And do they use the IMAP protocol or POP3?

They are all IMAP with mail.com. |The folder also appears on Apple's Mail, but not on Mail.com's webmail site. Spotlight found nothing on this Mac under RegMail, butt hen it is useless and couldn't even find itself.

What do you mean in saying "it can't be deleted"?

Very weird. the only thing I can think of is to do with Music Streaming.

see this link: http://dsync.blogspot.co.uk/2012/10/tutorial-installing-your-own-subsonic.html

scroll down to locate this: - /lib/mail/regmail.txt Contains the format of the mail received by the user when they register.

but why there is a folder in TB is unknown at this point. Can you think of anything you have installed recently?

Click on folder, click on right button, select delete folder, confirm. Nothing happens. I don't use music streaming and haven't installed any 3rd party software since January, and I first noticed this in mid-Feb. Still baffled

Your email provider, mail.com, seems to be part of the 1&1 organization. I have a gmx.co.uk email account with them, which has also suddenly acquired a RegMail folder.

If I try to delete it, there is an error message saying that the folder cannot be renamed, which is geek-speak for saying we users are not permitted to delete it.

This needs to be taken up with 1&1 I think. I don't go the website very often so may well have missed any announcements about it. I'm fully expecting to find it is some new extra feature of the account which I'll never use.

Someone else has also seen it. http://superuser.com/questions/1043556/odd-new-regmail-mailbox-in-apple-mail-9-2

I'm not experiencing this issue, but I would be curious to know what put it there and what is it's purpose. what information is being put in that folder and who has access to it. Definately a question to ask the hosting company.

I raised a ticket with gmx and they denied all knowledge of the RegMail folder. They can't see it in my folders so assert that my mail client put it there.

That will be along with the Queue and OUTBOX folders which also only appear in this particular account. ;-)

To get any sense, I'd probably need to go on beyond the first-tier support layer, but can't see that it's important enough to chase up. I'll keep an eye on the folder and see if which, if any, messages are put into it. So far it is empty, but is writeable; I have saved a message to it and then deleted this message.

My experience matches those reported on superuser; you can delete it (in the filesystem) and it re-appears (as do OUTBOX and Queue.)

I'm a little sad to read on superuser that google have acquired gmx; can anyone corroborate this?

Mail.com ignored my question and cut 'n' pasted this tat:

Please apologize the delayed reply. We have a higher than usual ticket volume.

In order to be able to use Outlook/Thunderbird you need the IMAP/POP3. These features are Premium features only which is the reason why we are not able to provide you with further Support (not included in the free mail.com service). However, as this folder only appears within your Outlook we would rather recommend you to contact Outlook Support.

Thank you for your understanding.

Best regards, Your mail.com Support Team.

I don't use Outlook because I have a Mac